Since instantaneousness and high-quality images are obtained, electrophotography has been used extensively in the fields of on-demand printers, copiers, various printers, and the like.
As photoreceptors serving as the core of electrophotography, use is being made of photoreceptors employing an organic photoconductive material which has advantages such as non-polluting properties, ease of film formation, and ease of production.
Photoreceptors employing an organic photoconductive material include: a so-called dispersion type photoreceptor containing photoconductive fine particles disperses in a binder resin; and a multilayer type photoreceptor having superposed layers including a charge-generating layer and a charge-transporting layer. The multilayer type photoreceptor has the following advantages: the multilayer type photoreceptor can be obtained as a high-sensitivity photoreceptor by using a charge-generating material having a high efficiency in combination with a charge-transporting material having a high efficiency; there is a wide choice of material and highly safe photoreceptors are obtained; and, since the photosensitive layer can be easily formed by coating, the multilayer type photoreceptor has high productivity and is advantageous also in view of cost. Therefore, the multilayer type photoreceptors are the mainstream of photoreceptors, and have been diligently developed and put to practical use.
Of these, for the reasons that the form is flexible and freedom at disposition in an apparatus is large, an endless belt-shaped photoreceptor obtained by linking a sheet-shaped photoreceptor at end parts thereof has been used by preference. Also, for the reasons that a photoreceptor having a wide area can be easily produced and the photoreceptor is easily changed inexpensively, a photoreceptor in a form where a sheet-shaped photoreceptor is wound on a drum has been used by preference (e.g., see Patent Documents 1 and 2).
Since the electrophotographic photoreceptor is repeatedly used in an electrophotographic process, i.e., in a cycle including charging, exposure, development, transfer, cleaning, erase, and the like, the photoreceptor is deteriorated by various stresses during the process. The deterioration includes such chemical and electrical deterioration that strongly acidic ozone and NOx generated from a corona charging unit used as a charging unit cause chemical damage to the photoreceptor and a photosensitive layer composition is decomposed by the flow of carriers formed by image exposure or erasing light or is decomposed by external light. Moreover, as deterioration other than the above, there is mechanical deterioration such as wear of the surface of the photosensitive layer, generation of scratches thereon, and exfoliation of a film caused by sliding with cleaning blades and magnetic brushes, contact with a transfer member or paper, and the like. Also, particularly in the endless belt-shaped photoreceptor, cracks may be generated on the surface of the photoreceptor owing to flexure and tension generated at the time when the photoreceptor repeatedly passes around rollers which constitutes a belt unit. In particular, the damage generated on the surface of the photosensitive layer is prone to appear on an image and directly impairs quality of the image, so that the damage becomes a large factor of limiting the life of the photoreceptor. Namely, in order to develop a long-life photoreceptor, it is an essential condition to increase mechanical strength together with enhancement of electrical and chemical durability.
Moreover, the sheet-shaped photoreceptor is difficult to secure adhesiveness between a conductive substrate and the photosensitive layer owing to the flexibility of the photoreceptor, so that there is a concern of exfoliation of the photosensitive layer.
In the case of a general photoreceptor having no functional layer such as a surface protective layer, the layer receiving such loads is the photosensitive layer. The photosensitive layer is usually composed of a binder resin and a photoconductive material, and the binder resin substantially determines the strength. However, since the doping amount of the photoconductive material is considerably large, a sufficient mechanical strength has not yet been realized.
As binder resins for the photosensitive layer, use has been made of vinyl polymers such as poly(methyl methacrylate), polystyrene, and polyvinyl chloride and copolymers thereof, thermoplastic resins such as polycarbonate, polyester, polysulfone, phenoxy, epoxy, and silicone resins, and various thermosetting resins. Of the numerous binder resins, polycarbonate resins have relatively excellent performance and thus various polycarbonate resins have hitherto been developed and put to practical use (e.g., see Patent Documents 3 to 6).